1. Field of the Invention
The invention relates to a control device for a power train and, more particularly, to a technology of compensating the torque transmitted from a power train to a wheel.
2. Description of the Related Art
Hybrid vehicles which have an internal combustion engine and a rotary electric machine as drive sources are known. In such a hybrid vehicle, the internal combustion engine and the rotary electric machine are selectively used in accordance with the state of run of the vehicle. For example, during high-speed running or the like, the vehicle runs by mainly using the internal combustion engine. During intermediate-to-low speed running or the like, the vehicle runs by mainly using the rotary electric machine. One of such hybrid vehicles is equipped with a differential mechanism that functions as a continuously variable transmission by using a rotary electric machine.
Japanese Patent Application Publication No. 2006-22933 (JP-A-2006-22933) discloses a control device for a vehicular driving device equipped with a stepless speed change portion that has a differential mechanism which distributes output of the engine to a first electric motor and to a transmission member, and that also has a second electric motor which is provided on a power transmission path from the transmission member to driving wheels, and that is capable of operating as an electrical continuously variable transmission. This control device includes an engagement device that is provided in a differential mechanism and that switches the stepless speed change portion between a stepless speed change state in which an electrical stepless speed change operation is possible and a stepped speed change state in which an electrical stepless speed change operation is not performed, and a torque reduction control portion that reduces at least one of the output torque of the engine, the output torque of the first electric motor and the output torque of the second electric motor, when the engagement device switches the stepless speed change portion from the stepless speed change state to the stepped speed change state.
According to the control device for a vehicular driving device described in the foregoing patent application publication, when the engagement device is engaged for the switching from the stepless speed change state to the stepped speed change state, at least one of the output torque of the engine, the output torque of the first electric motor and the torque of the second electric motor is reduced by the torque reduction control portion. Therefore, the switch shock involved in the switching from the stepless speed change state to the stepped speed change state is restrained.
However, in the control device described in JP-A-2006-22933, the output torque of the drive source is reduced at the time of the switching from the stepless speed change state to the stepped speed change state. Therefore, if the output torque of the rotary electric machine is insufficient at the time of the switching from the stepless speed change state to the stepped speed change state, the torque transmitted to the wheels, that is, the torque used for the running of the vehicle, declines. As a result, torque becomes discontinuous.